I have a 2001 Ford Taurus Ses flex fuel ohv 3.0. For the past few weeks (almost a month) it has been knocking/pinging under moderate and heavy acceleration. It literally sounds like a cold diesel engine knock. It does not happen every time, however when it does, if I back out of the accelerator even just a little it quiets down. I have drive over 150 miles a day, and have tried many different brands and grades of gas, I have tried e85 nothing seems to change it. I am 100 percent positive based on the flex fuel symbol on the car and the vin it is flex fuel. Under light acceleration, at idle or even cruising down the highway at 80mph it runs smooth no noises at all, step on it at a stop light (normal acceleration or to pass) and it literally sounds like a old cold diesel engine, it knocks rather loud, back off just a touch it quiets back out. I'm rather baffled. Spark plus have been checked, and gapped(they aren't terribly old), with new plug wires and coil pack (all motorcraft), Newer motorcraft fuel pump. No check engine lights or stored codes. Thanks in advance for your help.

My 2000 Flex Fuel Taurus does the same thing; I just tune it out, since I know it has a vacuum leak and it runs fine other than the sound. Plus, if it was a horrible sign of impending disaster, somebody would've said something to me after the many oil changes, tire replacements and other things I've had done to it.

I'd say if it's not the engine, it could be the water pump or power steering pump making that noise; other than the alternator, those are the only things under the hood that spin when it's running, besides the engine, of course.